A typical Josephon junction includes two separated superconductors, an insulator (called "gap") connecting them, and an external current. Electrons within superconductors are same in size and opposite directions in momentum, and they may form a stable shape when their cooper pair are consisted of two electrons having opposite spins. However, the cooper pair may break and the excited electrons or holes created due to the breakage are referred as quasi-particles.
When two superconductors are near enough, the cooper pair located at one superconductor exits the gap toward the other superconductor, and this is referred as a Josephon tunneling. Movement of the cooper pair is called a super current having no resistance. If the external current is less than a critical current, the external current appears as a super current that is movement of the cooper pairs. However, if the external current is greater than the critical current, current (called "common current") due to the tunneling of quasi-particles other than the super current is generated because the cooper pairs will break, and accordingly there exists a resistance and it will cause a voltage to occur.